The present invention relates to an electrochemical energy generating apparatus and a method of driving the apparatus.
Because of the high energy density of fuel, fuel cells are expected as next-generation batteries for not only electric vehicles but also mobile apparatuses such as notebook type personal computers and mobile phones, and research and development thereof are being vigorously conducted in many research institutes and companies.
Especially, the so-called polymer solid electrolyte type fuel cells, in which a polymer solid electrolyte is used to form an ion-permeable electrolyte membrane, are considered to be suitable for application to batteries for electric vehicles and mobile apparatuses, because of their comparatively low operating temperatures.
The polymer solid electrolyte type fuel cells generally have a configuration in which electrodes are provided on both sides of an ion-permeable electrolyte membrane.
In addition, as a fuel for use in fuel cells, there have been investigated various chemical substances, such as hydrogen and aqueous solutions of alcohols represented by methanol.
Among others, the fuel cell in which a fuel including an aqueous solution of an alcohol such as methanol is supplied directly to an electrode is deemed as most promising as a fuel cell for mobile apparatuses, from the viewpoints of smaller system size, flexibility of fuel cartridge and the like (refer to, for example, Patent Document 1). Such form of fuel cell will hereinafter be referred to as the direct alcohol fuel supply type fuel cell.
As one example of fuel cell, there may be mentioned a configuration in which a pair of electrodes (a fuel electrode and an air electrode) are formed respectively on both sides of an electrolyte membrane. In this configuration, a methanol-water mixed liquid is supplied as a fuel to the fuel electrode, whereas oxygen gas or air is supplied as air gas to the air electrode, so as to generate electric power through the following electrochemical reactions.Fuel electrode: CH3OH+H2O→CO2+6H++6e−Air electrode: 3/2O2+6H++6e−→3H2OCell reaction: CH3OH+3/2O2→CO2+2H2O
In short, methanol and water react with each other in a molar ratio of 1:1 on the fuel electrode side. Therefore, the mixing ratio of methanol and water in the aqueous methanol solution supplied to the fuel electrode side is desirably set to the same molar ratio as just-mentioned.
[Patent Document 1]
Japanese Patent Laid-open No. Hei 3-208260 (page 3, right bottom column, lines 1-6; FIG. 1)
However, the direct methanol fuel supply type fuel cell is known to have the problem of crossover. The crossover is a phenomenon in which the methanol supplied to the fuel electrode is not completely consumed in the reaction at the fuel electrode but, in part, permeates through the electrolyte membrane to reach the air electrode, to be consumed at the air electrode.
Due to the crossover, the amount of methanol to be inherently used for power generation is reduced, and a reverse electromotive force is generated, leading to a lowering in cell voltage. As a result, the fuel cell is lowered in output density and energy density.
It is also known that where a high-concentration aqueous methanol solution such as a methanol-water mixture with a molar ratio of 1:1 is used as the fuel, the crossover is conspicuous, whereby power generation characteristics would be considerably lower.
In order to solve this problem, development of an electrolyte membrane through which methanol permeates with difficulty and research and development for improving the fuel cell system have been being conducted.
For example, there is a method in which a low-concentration aqueous methanol solution prepared through sufficient dilution of methanol with water is supplied to the fuel electrode. Although the crossover is alleviated by this method, the amount of methanol in the fuel cartridge is greatly reduced, spoiling the high energy density, which is the characteristic inherent in the fuel cell.
Meanwhile, there has been disclosed a method in which an accessory is disposed in the exterior of a fuel cell, the water produced attendant on power generation at the air electrode is recovered by the accessory, high-concentration methanol is diluted with the recovered water, and the resulting aqueous methanol solution is supplied to the fuel electrode side (refer to, for example, Japanese Patent Laid-open No. 2004-146370). Since the water recovered at the air electrode is used in this method, it is possible to reserve high-concentration methanol in the fuel cartridge and, therefore, to make the fuel cartridge smaller in size. However, the need to dispose the accessory in the exterior of the fuel cell makes it difficult to render the fuel cell system smaller in size.